Massive beamforming is regarded as an important technical component for 5G wireless communications. With massive beamforming, hundreds of antenna elements are arranged to be used at the base station (BS) as opposed to only a few antennas in previous systems.
Many beamforming concepts rely on the base station (BS) using a grid-of-beams (GoB) approach. One such approach is that the BS transmits reference signals in a number of beams, herein referred to as beam reference signals (BRS:s), in order to enable a user terminal, user equipment (UE), to measure which beam that is the best/most suitable. Such measurements could be based on, e.g., reference signal received power (RSRP). The UE then reports an index for the best beam, or indices to a number of the best beams to the BS. The RSRP for the corresponding beams could also be reported to the BS. Based on these reports, the BS can decide which beam or beams to use for the data transmission.
The GoB typically consists of a number of predetermined fixed beams with beamforming weights obtained from a, possibly oversampled, DFT (Discrete Fourier Transform) matrix. In order not to lose any information in the data collected by the antenna elements, the number of beams in the GoB should be at least as many as the number of antenna elements. This means that the number of beams in the GoB of a potential massive beamforming system at least could be several hundreds.
With many beams in the GoB, a high number of BRS:s may be required. This could lead to pilot contamination, high consumption of radio resource elements for the pilots, and comprehensive measurement procedures in the UE in order to estimate the best beam or beams to be used for data transmission.
One solution to this problem is to transmit BRSs only in a few beams in the GoB. If the directions to all active UE:s are known, the UE:s are not moving, and no new UE:s enter the system, this could be a viable approach. However, problems will occur if new UE:s enter the system and/or established propagation paths suddenly are obstructed due to, e.g., a UE moving behind a building. In such cases, the direction to the new UE or the new dominating propagation path cannot be estimated by means of BRSs if no BRSs are transmitted in those directions. This may lead to that active transmissions beams are lost and that new active transmission beams cannot be established for new UE:s.
There is thus a need for a wireless communication node that is arranged to handle abrupt changes in dominating propagation paths for existing UE:s and the appearance of new UE:s in a more efficient and reliable manner than previously known.